Stockholders' Equity (Tables)

A summary of the activities related to the Company’s stock option plans is as follows:



			Options Outstanding
	Shares Available for Grant		Number of Shares		Weighted- Average Exercise Price (per share)		Weighted-Average Remaining Contractual Term (in years)	Aggregate Intrinsic Value (in thousands)
Balances as of December 31, 2018	8,699,941		20,479,278		$	89.61
Granted	(619,636	)	619,636		319.43
Exercised	—		(593,294	)	38.07
Expired	—		(21	)	4.27
Balances as of March 31, 2019	8,080,305		20,505,599		$	98.04	5.68	$	5,312,077
Vested and exercisable as of March 31, 2019			20,505,599		$	98.04	5.68	$	5,312,077

A summary of the amounts related to option exercises, is as follows:



	Three Months Ended
	March 31, 2019		March 31, 2018
	(in thousands)
Total intrinsic value of options exercised	$	180,842	$	277,910
Cash received from options exercised	22,972		56,335

The following table summarizes the assumptions used to value option grants using the lattice-binomial model and the valuation data:



	Three Months Ended
	March 31, 2019			March 31, 2018
Dividend yield	—		%	—		%
Expected volatility	41		%	40		%
Risk-free interest rate	2.74		%	2.61		%
Suboptimal exercise factor	3.07			2.80
Weighted-average fair value (per share)	$	163		$	124
Total stock-based compensation expense (in thousands)	$	101,200		$	68,395
Total income tax impact on provision (in thousands)	$	21,628		$	14,691